1. Field of the Invention
The present invention relates to a display in which the angular output range of light is controllable so that the display can be switched between a wide angle viewing mode and a narrow angle viewing mode, and which can further be switched to a multiple view directional display mode.
2. Description of the Related Art
Electronic display devices such as, for example, monitors used with computers and screens built in to mobile telephones and other portable information devices, are usually designed to have as wide a viewing angle as possible, so that an image displayed by the device can be seen from many different viewing positions. However, there are some situations where it is desirable for an image displayed by a device to be visible from only a narrow range of viewing angles. For example, a person using a portable computer in a crowded train might want the display screen of the computer to have a small viewing angle so that a document displayed on the computer screen cannot be read by other passengers on the train. For this reason, there has been considerable effort put in to developing display devices which are electrically switchable between two modes of operation—in a ‘public’ display mode they have a wide viewing angle for general use, but they can be switched to a ‘private’ display mode in which they have a narrow viewing angle so that private information can be displayed in public places without being visible to people other than the user of the device.
Another application of such a display may be as a display in a motor vehicle. The viewing angle of the display could be controlled such that the passengers are unable to see the display or such that the driver is unable to see the display. Alternatively, the viewing angle could be controlled in order to reduce the reflections of the display in the windscreen and the windows—so that, for example, the viewing angle could be reduced at night-time or in low light conditions. A brightness sensor could be provided to allow automatic switching between a wide viewing angle and a narrow viewing angle, and also to allow automatic control of the brightness of the display.
Displays are also known which display two or more images simultaneously, with each image being displayed in a different direction from the other image, and such displays are known as multiple view directional displays. The images may be still images or moving images (i.e., a sequence of images).
One type of a multiple view directional display is an autostereoscopic 3-D display, which displays two images that are the left eye image and right eye image of a stereoscopic image pair. The two images are displayed such that the left eye image is directed to the left eye of an observer and the right eye image is directed to the right eye of an observer, whereby the observer perceives a full 3-D image.
Another type of a multiple view directional display is a dual view display. A dual view displays two (or more) images, such that one image is directed to one observer and another image is directed to another observer. The images are independent from one another, and may be completely unrelated to one another such that the two observers see completely different images.
A number of devices are known which restrict the range of angles or positions from which a display can be viewed.
U.S. Pat. No. 6,552,850 describes a method for the display of private information on an automatic teller machine (ATM). Light emitted by the machine's display has a fixed polarization state, and the machine and its user are surrounded by a large screen of sheet polarizer which absorbs light of that polarization state but transmits light of the orthogonal polarization state. Passers-by can see the user and the machine, but cannot see information displayed on the machine's screen.
One known element for controlling the direction of light is a ‘louvered’ film that consists of alternating transparent layers and opaque layers provided in an arrangement similar to a Venetian blind. Such a film is shown schematically in FIG. 1. The film operates on the same principle as a Venetian blind, and it allows light to pass through it when the light is traveling in a direction parallel to, or nearly parallel to, the opaque layers, as shown by ray 30 in FIG. 1. However, light traveling at large angles relative to the plane of the opaque layers is incident on one of the opaque layers and is absorbed, as shown by ray 31 in FIG. 1. The layers may be perpendicular to the surface of the film, as shown in FIG. 1, or they may be arranged at some other angle relative to the surface of the film.
Louvered films of this type may be manufactured by stacking many alternating sheets of transparent material and opaque material and then cutting slices of the resulting block perpendicular to the layers. This method has been known for many years and is described in, for example, U.S. Pat. Nos. 2,053,173; 2,689,387 and 3,031,351.
Other manufacturing methods are known. For example, U.S. Pat. No. RE27,617 describes a process where a louvered film is cut continuously from a cylindrical billet of stacked layers. U.S. Pat. No. 4,766,023 describes how the optical quality and mechanical robustness of the resulting film can be improved by coating with a UV-curable monomer and then exposing the film to UV radiation. U.S. Pat. No. 4,764,410 describes a similar process where a UV-curable material is used to bond the louver sheet to a covering film.
Other methods exist for making films with similar properties to the louvered film. For example, U.S. Pat. No. 5,147,716 describes a light-control film which contains many elongated particles which are aligned in the direction perpendicular to the plane of the film. Light rays which make large angles to this direction are therefore strongly absorbed, whereas light rays propagating in this direction are transmitted.
Another example of a light-control film is described in U.S. Pat. No. 5,528,319. This film has a transparent body in which are embedded opaque regions that extend generally parallel to the plane of the film. The opaque regions are arranged in stacks, with each stack being spaced from a neighboring stack. The opaque regions block the transmission of light through the film in certain directions while allowing the transmission of light in other directions.
The prior art light control films may be placed either in front of a display panel or between a transmissive display panel and its backlight, to restrict the range of angles from which the display can be viewed. In other words, the prior art light control films make a display ‘private’. However, none of the prior art light control films enables the privacy function to be switched off to allow viewing from a wide range of angles.
There have been reports of a display which can be switched between a public mode (with a wide viewing angle) and a private mode (with a narrow viewing angle). For example, U.S. Patent Publication No. 2002/0158967 suggests that a light control film could be movably mounted on a display so that the light control film either may be positioned over the front of the display to provide a private mode or may be mechanically retracted into a holder behind or beside the display to provide a public mode. This method has the disadvantage that it contains moving parts which may fail or be damaged in use, and which add bulk to the display.
A method for switching a display panel from public to private mode with no moving parts is to mount a light control film behind the display panel, and to place a diffuser which can be electronically switched on and off between the light control film and the panel. When the diffuser is inactive, the light control film restricts the range of viewing angles and the display is in a private mode. When the diffuser is switched on, the light with a narrow angle range output from the light control film is incident on the diffuser, and the diffuser acts to increase the angular spread of the light—that is, the diffuser cancels out the effect of the light control film. Thus, the display is illuminated by light traveling at a wide range of angles and the display operates in a public mode. It is also possible to mount the light control film in front of the panel and place the switchable diffuser in front of the light control film to achieve the same effect.
Switchable privacy devices of the above type are described in U.S. Pat. Nos. 5,831,698; 6,211,930; and 5,877,829. They have the disadvantage that the light control film always absorbs a significant fraction of the light incident upon it, whether the display is in public mode or private mode. The display is therefore inherently inefficient in its use of light. Furthermore, since the diffuser spreads light through a wide range of angles in the public mode, these displays are also dimmer in public mode than in private mode (unless the backlight is made brighter when the device is operating in public mode to compensate).
Another disadvantage of these devices relates to their power consumption. Such devices often use a switchable polymer-dispersed liquid crystal diffuser which is not diffusive when no voltage is applied across the liquid crystal layer and which is switched on (into the diffusive state) by applying a voltage. Thus, to obtain the public mode of operation it is necessary to apply a voltage across the diffuser so that the diffuser is switched on. More electrical power is therefore consumed in the public mode than in the private mode. This is a disadvantage for mobile devices which are used for most of the time in the public mode and which have limited battery power.
Another method for making a switchable public/private display is described in U.S. Pat. No. 5,825,436. The light control device in this patent is similar in structure to the louvered film described above. However, each opaque element in a conventional louvered film is replaced by a liquid crystal cell which can be electronically switched from an opaque state to a transparent state. The light control device is placed in front of or behind a display panel. When the cells are opaque, the display operates in a private mode; when the cells are transparent, the display operates in a public mode.
One significant disadvantage of this device is the difficulty and expense of manufacturing liquid crystal cells with an appropriate shape. A second disadvantage is that, in the private mode, a ray of light may enter at an angle such that it passes first through the transparent material and then through part of a liquid crystal cell. Such a ray will not be completely absorbed by the liquid crystal cell and this may reduce the privacy of the device.
Japanese Patent Publication No. 2003-233074 describes a display having a switchable viewing angle. This uses an additional LC panel, which is segmented. Different segments of the additional LC panel modify the viewing characteristics of the associated areas of the display in different ways, with the result that the whole display panel is fully readable only from a central viewing position.
U.K. Patent Application No. 0320363.5 describes switchable privacy devices based on louvers, which operate only for one polarization of light. The louvers are switched on and off either by rotating dyed liquid crystal molecules in the louver itself or by rotating the plane of polarization of the incident light using a separate element.
U.K. Patent Application No. 0408742.5 describes a switchable privacy device constructed by adding one or more extra liquid crystal layers and polarizers to a display panel. The intrinsic viewing angle dependence of these extra elements can be changed by switching the liquid crystal electrically in the well-known way.
U.K. Patent Application No. 0401062.5 describes a display that is switched between a public mode and a private mode by using two different backlights which generate light with different angular ranges.
GB 2 410 116, which was published after the priority date of this application, relates to a display device that is switchable between a public display mode and a private display mode. This is achieved by providing two backlights with different angular output ranges, and selecting the narrow output backlight to obtain a private display mode or selecting the wide output backlight to obtain a public display mode. In one embodiment, the backlight is formed of two inter-digitated illumination systems, with each illumination system being illuminated either by a visible light source or by a UV light source. Each illumination system includes a phosphor sheet, and so emits visible light with a wide angular range when the respective UV light source is illuminated. If the visible light source is illuminated, however, the illumination system emits light with a narrow angular range. The backlight can therefore operate in various modes—(1) if both visible light sources are illuminated it emits visible light with a narrow angular range over its entire area; (2) if both UV sources are illuminated, it emits visible light with a wide angular range over its entire area; (3) if just one visible [or UV] source is illuminated, it emits visible light with a narrow [wide] angular distribution over the area corresponding to one of the inter-digitated illumination systems.
GB 2 405 544 is directed to a polarization-dependent light control structure in which the light control structure is arranged to act as a parallax barrier for light having a particular plane of polarization. Depending on the state of a switchable half-wave plate, the light control structure may be rendered ineffective, thus providing a public 2-D mode or it may act as a parallax barrier thereby providing a 3-D display mode. Furthermore, if the half-wave plate is switched so that the light control structure is enabled, a private 2-D display mode may be obtained by displaying a single image on the display panel.